1. Technical Field
Embodiments relate to a nonvolatile memory device and a method of manufacturing the same.
2. Description of the Related Art
Nonvolatile memory devices include semiconductor memory devices that maintain stored data even when the power supply is interrupted. Phase-change random access memory (PRAM) can store data according to a resistance state of a phase-change material pattern. A memory cell storing binary data of a PRAM may include a switching device, a lower electrode, a phase change material pattern, and an upper electrode. The switching device may be formed on a semiconductor substrate and the lower electrode, the phase change material pattern, and the upper electrode may be formed on the switching device. Chalcogenide of GeSeTe (GST) system may be used as the phase change material pattern. The lower electrode may heat the phase change material pattern. A portion of the phase change material pattern, or all of the phase change material pattern, may be converted from a crystalline state to an amorphous state, or from an amorphous state to a crystalline state. This conversion depends on the degree of heating of the phase change material pattern by the lower electrode, and a resistive value of the phase change material pattern.
Specifically, a PRAM may apply a set pulse and a reset pulse to store binary data. The set pulse is needed to convert the phase change material pattern to a crystalline state. The set pulse applies a temperature higher than a temperature required to change the phase change material pattern to a crystalline state to the phase change material pattern for several nanoseconds through the lower electrode.
The reset pulse is needed to convert the phase change material pattern to an amorphous state. The reset pulse applies a temperature higher than a temperature required to change the phase change material pattern to the amorphous state to the phase change material pattern for several tens of nanoseconds through the lower electrode.
Conventionally, a temperature of the phase change material pattern affected by the reset pulse is higher than a temperature of the phase change material pattern affected by the set pulse. Since temperature rise is effected by Joule heating caused by an electrical current, the current flowing through the phase change material pattern when the set pulse is applied is smaller (for a corresponding lower temperature) than a current flowing through the phase change material pattern when the reset pulse is applied (when the temperature should be higher).
Binary data may be stored and read depending on a resistance state of the phase change material pattern. The amorphous state material may have a resistivity higher than that of the crystalline state material, and thus the phase change material pattern may have a higher resistance for a given pattern size and shape when the material is in the amorphous state. The crystalline state is referred to as a set state and the amorphous state is referred to as a reset state.
When reading data, the reading operation can maintain a high speed when a resistance (hereinafter, referred to as a set resistance) of the phase change material pattern in the crystalline state (i.e., the set state) is small. When storing data, the storing operation can maintain a high speed when a reset current for changing the phase change material pattern to an amorphous state (i.e., the reset state) is small. Thus, in a PRAM, a reset current should be reduced for a fast memory operation and a set resistance should be reduced for a fast writing operation.
A phase change material pattern used in a PRAM is mainly material of GST system. When a voltage is applied to the phase change layer, the phase change layer shows a negative resistance characteristics and a resistance of the phase change material pattern is abruptly reduced. A PRAM is a memory device for storing binary data. A PRAM easily discriminates binary data because a ratio of a resistance according to a state of a binary data. A PRAM may not require a high voltage.